The present invention relates to a device for carrying out delta-sigma modulation of an audio signal which has been caused to undergo oversampling, and more particularly to a delta-sigma modulation device for correcting signal distortion produced when delta-sigma modulated signal is amplified by amplifier, and a signal amplifier apparatus comprising such delta-sigma modulation device.
Hitherto, as a method of improving S/N ratio in the audible band of audio signal, delta-sigma modulation system is known. In this delta-sigma modulation system, by the noise shaping technology, e.g., PCM digital signal of 16 bits or more is converted into quantization signal having lower number of bits from one bit to several bits to have ability to reproduce it into analog signal by local D/A converter of several bits.
Here, reproduction of audio signal utilizing the delta-sigma modulation system will be explained. A reproducing apparatus 9 which reproduces audio signal comprises, as shown in FIG. 1, a delta-sigma modulator 90, a Pulse Width Modulator 91, a switching module 92, a LPF (Low Pass filter) 93, and a speaker 94.
The delta-sigma modulator 90 comprises, as shown in FIG. 2, an input terminal supplied with an audio signal, a subtracter 191 for carrying out subtraction between audio signal and delay signal, an integrator 192 for integrating an output of the subtracter 191, a quantizer 193 for quantizing an output of the integrator 192 into a digital signal having lower number of bits, and a delay circuit 194 for delaying output S1 of the quantizer 193 by one clock (operating clock of the quantizer) to input it as a delay signal to the subtracter 191. In delta-sigma modulator 90, there is generated an audio signal distributed so that quantization noise is caused to be less in the Nyquist band which is the band having a frequency up to one half of sampling frequency of input audio signal and quantization noise is caused to be many in the frequency band more than the Nyquist band. For example, input audio signal is digital audio data having 24 bits and sampling frequency of 768 kHz (=48 kHzxc3x9716), and output audio signal is digital signal having 6 bits and sampling frequency of 768 kHz. The output audio signal S1 is inputted to the Pulse Width Modulator 91. The Pulse Width Modulator 91 carries out Pulse Width Modulation with respect to inputted audio signal.
The Pulse Width Modulated audio signal S2 is amplified by the switching module 92 into a pulse signal of a predetermined magnitude, and an audio signal of the audible band is outputted from the speaker 94 through the LPF 93.
Since the switching module 92 is ordinarily operative at a power supply voltage higher than input voltage, e.g., 20Vxcx9c50V, distortion takes place in an output signal. Moreover, the output signal also experiences influence of counter-electromotive force by the coil of the LPF 93 or the speaker 94 so that it is difficult that there results ideal waveform.
For example, as shown in FIG. 3, in the above-described reproducing apparatus 9, signal S3 amplified by the switching module 92 is such that delay of signal takes place by Trise at the rising and by Tfall at the falling with respect to the signal S2 which has been caused to undergo Pulse Width Modulation by the Pulse Width Modulator 91. As a result, there takes place the problem that pulse width of the signal S2 and that of the signal S3 are different from each other. There is the problem that ringing takes place at the rising and the falling in the signal S3 so that waveform like the signal S2 cannot be obtained.
Further, in the class D amplifier which drives the speaker 94 by signal S3 amplified by the switching module 92, since distortion of the signal S3 results in distortion of audio signal as it is so that such distortion is outputted from the speaker 94, there is the problem that the audio characteristic such as distortion ratio and/or S/N ratio, etc. is deteriorated.
In order to eliminate this distortion, a class D power amplifier as described in, e.g., the Japanese Patent Application Laid Open No. 214259/1997 publication is proposed. In this class D amplifier, waveform distortion by power supply ripple taking place at the power switch and/or waveform distortion by overshoot or ringing, etc. are suppressed.
This class D amplifier comprises a delta-sigma modulation unit as described above, a power switch for power-amplifying an output signal of the delta-sigma modulation unit, a first LPF for smoothing an output of the power switch, a delay for delaying an output signal of the delta-sigma modulation unit, an attenuator for attenuating amplitude of an output signal of the power switch, an adder for calculating difference between an output signal of the delay and an output signal of the attenuator, a second LPF for smoothing an output signal of the adder, and a selector circuit for selecting reference level of a quantizer on the basis of an output signal of the second LPF. Input signal is converted into binary signal at the delta-sigma modulation unit. This binary signal is power-amplified at the power switch, and is smoothed at the first LPF. The signal thus obtained is delivered to load. On the other hand, the delay delays an output of the delta-sigma modulation unit. The attenuator attenuates amplitude of an output signal of the power switch equivalently to amplitude of an output signal of the delay. Further, the adder calculates difference between an output signal of the delay and an output signal of the attenuator to smooth it at the second LPF to thereby detect waveform distortion taking place at the power switch. The selector circuit selects reference level of 1 bit quantizer in accordance with this waveform distortion quantity. As a result, the delta-sigma modulation unit converts input signal into binary signal so as to cancel waveform distortion. In this way, waveform distortion is cancelled.
Since the class D power amplifier described in the above-mentioned publication serves to change the reference level of the quantizer, in the case where a signal of high level is inputted to the quantizer, there is the problem that signal is clipped. Moreover, in the class D power amplifier, in the case where level of a signal inputted to the quantizer is suppressed to be low in order not to cause this clip, there is the problem that S/N ratio is deteriorated.
In addition, in the class D amplifier adapted for driving speaker by signal amplified by the switching module, since distortion of signal results in distortion of audio signal as it is so that it is outputted from the speaker, there is the problem that the audio characteristic such as distortion ratio and/or S/N ratio, etc. is deteriorated.
An object of the present invention is to provide a novel delta-sigma modulation device and a novel signal amplifier apparatus which can solve problems that conventional apparatuses as described above have.
Another object of the present invention is to provide a delta-sigma modulation device which corrects in advance distortion taking place at the switching module, and a signal amplifier apparatus using such delta-sigma modulation device.
A further object of the present invention is to provide a signal amplifier apparatus which corrects in advance distortion taking place at the switching module by a feedback circuit.
A delta-sigma modulation device according to the present invention proposed in order to attain objects as described above is directed to a delta-sigma modulation device comprising a quantizer and operative to feed output or quantization error of this quantizer back to the input side as a feedback signal, the delta-sigma modulation device comprising a correction circuit for correcting output signal or quantization error of the quantizer to allow it to be a feedback signal, thus to correct, by the correction circuit, distortion taking place by circuit or circuits connected to the succeeding stage of this delta-sigma modulation device.
The delta-sigma modulation device according to the present invention installs (provides) correction circuit which corrects distortion that circuit or circuits connected to the succeeding stage from this delta-sigma modulation device generate within feedback path from output or quantization error of quantizer toward the input side to correct in advance distortion that circuit or circuits connected to the succeeding stage from this delta-sigma modulation device generate.
Another delta-sigma modulation device according to the present invention is directed to a delta-sigma modulation device comprising a quantizer and operative to feed output or quantization error of this quantizer back to the input side as a feedback signal, the delta-sigma modulation device comprising a correction circuit for correcting an output signal of the quantizer to deliver it to circuit or circuits connected to the succeeding stage of this delta-sigma modulation device, thus to correct, by the correction circuit, distortion taking place by the circuit or circuits of the succeeding stage.
This delta-sigma modulation device installs (provides) the correction circuit for correcting distortion that circuit or circuits connected to the succeeding stage from this delta-sigma modulation device generates between the quantizer and the circuit or circuits connected to the succeeding stage to correct in advance distortion that the circuit connected to the succeeding stage from this delta-sigma modulation device generate.
A signal amplifier apparatus according to the present invention proposed in order to attain the above-described objects comprises: a delta-sigma modulator including a quantizer and a correction circuit thus to correct output or quantization error of this quantizer by the correction circuit to feed it back to the input side as a feedback signal; a pulse width modulator for carrying out pulse width modulation of a signal outputted from the delta-sigma modulator, an amplifier for amplifying the PWM (Pulse Width Modulated) signal outputted from the pulse width modulator to obtain an output signal, detecting means for detecting distortion quantity at the amplifier from the PWM signal and the output signal, and a correction circuit for correcting output of the quantizer of the delta-sigma modulator in accordance with distortion quantity detected at the detecting means to allow it to be a feedback signal.
This signal amplifier apparatus is operative to compare a signal inputted to the amplifier and a signal outputted therefrom by the detecting means to detect rising time difference and falling time difference of those signals to calculate distortion quantity taking place at amplifier output by the characteristic of circuit or circuits connected to the stages succeeding to the amplifier from the rising time difference and the falling time difference which have been detected to feed this calculated distortion quantity back to the delta-sigma modulator.
Another signal amplifier apparatus according to the present invention comprises: a delta-sigma modulator; a correction circuit for correcting a signal outputted from the delta-sigma modulator on the basis of a correction signal; a pulse width modulator for carrying out pulse width modulation of a signal outputted from the correction circuit, an amplifier for amplifying the PWM (Pulse Width Modulated) signal outputted from the pulse width modulator to obtain an output signal; and detecting means for detecting distortion quantity at the amplifier from the PWM signal and the output signal to provide the correction signal corresponding to the detected distortion quantity.
This signal amplifier apparatus is operative to compare signal inputted to the amplifier and signal outputted therefrom by the detecting means to detect rising time difference and falling time difference of those signals to calculate distortion quantity taking place at the amplifier output by the characteristic of circuit or circuits connected to the stage succeeding to the amplifier from the rising time difference and the falling time difference which have been detected to feed a correction signal corresponding to the calculated distortion quantity hack to the correction circuit.
Still further objects of the present invention and practical merits obtained by the present invention will become more apparent from the embodiments which will be given below with reference to the attached drawings.